The present invention relates to a rubber composition, vulcanized rubber and to a tire. More specifically, it relates to a rubber composition which can be used as vulcanized rubber and a raw material of vulcanized rubber. This vulcanized rubber can be employed in tires and tire tread to improve the following characteristics in a well balance manner: cornering, braking and traction property on icy roads (on-ice performance), handling on wet roads (wet properties), and wear resistance on dry roads.
Since the prohibition of spike tires, a deal of research into the tread of studless tires improving braking and driving performance on icy roads (on-ice performance) where films of water can often occur, has been carried out.
On icy surfaces, water films decrease the coefficient of friction between studless tires and the icy surface. Thus the ability of the tread of the studless tire to remove water film and the edge effect of the tread can effect on-ice performance of the studless tire drastically. Thus there is a need to improve the ability to remove water film and the edge effect.
Japanese Patent No. 2,568,502 discloses a technique where the tread of tires is made of foamed rubber, and independent cells in the foamed rubber cause fine bumps and pores which improve the ability to remove water film and the edge effect. However, use of foamed rubber containing only independent cells fails to provide a level of improvement in on-ice performance able to fully satisfy market demands.
Japanese Patent Application Laid-open (JP-A) No. 8-85738 discloses the use of rubber compositions having excellent hydrophobicity and water repellent properties. However, as in Japanese Patent No. 2,568,502, these techniques fail to provide on-ice properties of a level able to satisfy market demands.
On the other hand, methods using short fiber loaded foamed rubber in tire tread, and providing micro grooves in the surface of tire tread are disclosed in JP-A No. 4-38207.
However, the short fibers curl due to heat shrinkage, get squeezed, and contort in the tire treads by brades which make sipes in the tread during vulcanization. For this reason, even when tire-tread wears, the short fibers, which are not parallel to the plane of friction, are not easily removed from the tire-tread, and the expected microgrooves are not formed effectively, resulting in an insufficient increase in on-ice coefficient of friction. Also, the removal of the short fibers from the tire tread depends largely on traveling conditions and the like. Therefore improvements in on-ice performance cannot be achieved with certainty. Further, the microgrooves tend to collapse when tires are exposed to heavy loads. In this case there is also the further problem of serious reductions in wear resistance.
Also, a tire has been disclosed in JP-A No.4-110212 in which hollow fibers are dispersed in the tire tread. This enables the water formed between the icy surface and the contacting plane of the tire tread to be removed by the hollow section of the hollow fibers. However, the hollow fibers tend to collapse through pressure, rubber streaming, certain temperatures and the like applied at the time of formation when being kneaded with rubber. Therefore, the hollow fibers do not maintain their hollow configuration and the general problem of insufficient water removal persists. The problem of significant reductions in wear resistance is also seen in this case.
The object of the present invention is to solve the above problems.
The object of the present invention is to provide a tire which exhibits excellent ability to remove water formed between the tire and the icy surface, a high coefficient of friction with icy surfaces, and the ability to produce well balanced improvements in cornering, braking and traction property on icy roads (on-ice performance) and handling stability on wet roads (wet performance) and wear resistance on dry roads, without depending on traveling conditions.
The object of the present invention is to provide a vulcanized rubber suitable for use in a structure such as tread of the tire which can suppress slipping on ice.
Another object of the present invention is to provide a rubber composition suitable for use as a raw material for the vulcanized rubber and the tire.
After a deal of inquiry and investigation, the inventors of the present invention have made the following findings. On-ice properties are significantly improved when elongated cells are formed in the tread. However, in this case, wear resistance is significantly reduced when foamed rubber is used as the tread. This reduction in wear resistance can be suppressed if butadiene rubber is incorporated, but handling stability on wet roads (wet property) lowers. If silica is further incorporated, this reduction in the wet property can be suppressed. As a result, a method of maintaining on-ice performance, wet property, and wear resistance in a well balanced way has been realized.
The present invention is based on the findings by the inventors and the means taken to solve the above problems are as set out below.
1) vulcanized rubber comprising a rubber matrix and elongated cells, wherein said rubber matrix comprises i) a rubber component containing 20 to 70 parts by weight of natural rubber and 30 to 80 parts by weight of polybutadiene rubber, ii) 5 to 55 parts by weight of carbon black per 100 parts by weight of the rubber component, and iii) 5 to 55 parts by weight of silica per 100 parts by weight of the rubber component, and
wherein said elongated cells are coated with a resin layer and have a ratio (L/D) of an average length (L) to average diameter (D) of at least 3.
2) The vulcanized rubber of 1) above in which the average foaming ratio is 3 to 40%.
3) A tire comprising:
a pair of bead sections;
a carcass which extends between the pair of bead sections and is formed in a toroidal shape;
a belt which reinforces the crown of said carcass; and tread,
wherein the tread contains the vulcanized rubber described in 1) or 2) above.
4) The tire described in 3) in which elongated cells are oriented along the circumferential direction of the tire.
5) A rubber composition comprising a rubber matrix and elongated resin,
wherein said rubber matrix comprises i) a rubber component containing 20 to 70 parts by weight of natural rubber and 30 to 80 parts by weight of polybutadiene rubber, ii) 5 to 55 parts by weight of carbon black per 100 parts by weight of the rubber component, and iii) 5 to 55 parts by weight of silica per 100 parts by weight of the rubber component, and all of which becomes the vulcanized rubber described in 1) or 2) above through vulcanization.
6) The rubber composition described in 5) above wherein the rubber matrix contains a foaming agent, and wherein the viscosity of the elongated resin becomes lower than the viscosity of the rubber matrix before the temperature of the rubber composition reaches the maximum vulcanizing temperature during vulcanization.
7) The rubber composition described in 5) or 6) above wherein the elongated resin comprises a crystalline polymer having a melting point lower than the maximum vulcanizing temperature.
8) The rubber composition described in any one of 5) to 7) in which the melting point of the elongated resin is 190xc2x0 C. or less.